It is known that, with respect to a free propeller of the same diameter, a propeller shrouded in a duct theoretically makes it possible to obtain a substantially equal thrust or pull, with a gain in power of the order of 30%.
In fact, the duct improves the yield of the propeller installed inside, with respect to a free propeller, for two reasons:
the circulation of the air through the duct creates a depression on the shroud and therefore a thrust of the fairing in its assembly, which is substantially equal to the thrust of the propeller itself;
the flow in the vicinity of the shroud being in depression downstream of the propeller, the flux does not contract, contrarily to what occurs downstream of a free propeller, which has for its consequence to increase the yield of the propeller, and this all the more so as the diffusion of the fluid stream is increased without stall in the shroud.
This is why, in numerous applications in which, in limited dimensions, it is question of creating a force of aerodynamic origin by a propeller, the solution of a shrouded propeller has proved more advantageous than that contributed by a free propeller.
Among such applications may be mentioned vertical take-off and landing aircraft, in which one or more vertical-axis shrouded propellers are integrated in the fixed wing or fuselage; vehicles with lift by air-cushion, of which the pressurized air generators blowing towards the ground are propellers housed inside fairings, themselves incorporated in the body of the vehicle; and, finally, variable-pitch fans, for example those incorporated in a gas conduit in order to create a considerable circulation of said gas in the conduit.
A particularly advantageous application has been made thereof to produce the tail rotor of helicopters.
It is known that, on such aircraft with lifting rotary wing, and in particular on mechanically driven mono-rotor helicopters, in order permanently to balance the counter torque on the fuselage resulting from the rotation of the rotary wing, and in order to control the aircraft on its yaw axis, an auxiliary rotor is provided, disposed in the vicinity of the end of the tail of the aircraft and exerting a transverse thrust which is adaptable to all the flight conditions. This auxiliary tail rotor therefore exerts on the aircraft a balance torque of direction opposite the counter torque of the main rotor to its rotation by the engine or engines, i.e. in fact of the same direction as the driving torque of the lifting rotary wing.
Controlled variations of this balance torque by controlling the pitch of the blades of the anti-torque auxiliary rotor also enable the pilot to control the course of the helicopter about its yaw axis.
However, and particularly on helicopters of low or average tonnage, the conventional anti-torque rotor constituted by a free propeller is particularly vulnerable to outside aggressions: it may touch the ground staff or touch the ground itself or any obstacle, all collisions which directly compromise the balance of the helicopter and its safety in flight.
It is particularly in order to avoid these serious drawbacks that Applicants have developed on helicopters of low and average tonnage, a multiblade tail rotor arrangement, shrouded inside the vertical stabilizer of these apparatus.
Such an installation is rendered possible and advantageous by the fact that the diameter of such a shrouded rotor may be relatively reduced with respect to that of a free rotor of equivalent efficiency.
Such arrangements of anti-torque shrouded rotors are for example described in U.S. Pat. Nos. 3,506,219, 3,594,097 and 4,281,966.
Of course, it is sought to obtain from this auxiliary rotor, under the optimum conditions of yield as far as the driving power is concerned, a sufficiently high maximum thrust to satisfy the most demanding flight conditions and, by controlling the pitch of the blades, it is provided to take only a part of this maximum thrust adapted to the other flight cases.
It is known that the lifting efficiency of the rotary wings is generally characterized, for stationary operational conditions, by a parameter known under the term of "figure of merit" which is the ratio between the minimum power for obtaining a given pull or thrust and the real power effectively measured.
For a shrouded propeller, the expression of this parameter is given by the following known formula: ##EQU1## in which FM is the figure of merit,
T the desired thrust or pull, PA0 P the necessary power to be furnished to the propeller, PA0 .rho. the density of the air, PA0 R the radius of the propeller, and PA0 .sigma. the coefficient of diffusion of the aerodynamic flux on the surface, this coefficient .sigma. being equal to the ratio S.infin./S, with S.infin. representing the surface of the flux at downstream infinite and S being the surface of the disc formed by the propeller in rotation. PA0 (a) said maximum relative camber increases virtually linearly from this value close to 0 to a value equal to 0.036 for a relative span equal to 0.845, passing through values 0.01 and 0.02 respectively for the relative spans 0.53 and 0.66, then increases from this value equal to 0.036 for the relative span equal to 0.845 up to a value equal to 0.038 for the relative span equal to 0.93, and, finally, is constant and equal to 0.038 between the relative spans 0.93 and 1; PA0 (b) the root of said blade presents evolutive profiles of which the maximum relative camber is negative and increases from a value substantially equal to -0.013 for a relative span equal to 0.40 to said value close to 0 for a relative span equal to 0.45; PA0 (c) between the relative spans 0.45 and 1, the evolution of the twist is at least substantially parabolic, with a minimum at the relative span of 0.86; PA0 (d) the axis of twist of said aerodynamically active part is parallel to the line of leading edge and to the line of trailing edge thereof and is distant from said line of leading edge by a distance approximately equal to 39% of the length of the chord of the profiles; PA0 (e) the twist of the root of said blade increases from said value close to 8.degree. for a relative span close to 0.38 to said value close to 12.degree. for the relative span equal to 0.45; PA0 (f) the maximum relative thickness of the profiles decreases linearly from a value close to 13.9% for a relative span equal to 0.40 to a value close to 9.5% for a relative span equal to 0.93, and is constant and equal to said value close to 9.5% between the relative spans respectively equal to 0.93 and 1.
In order to increase the figure of merit with fixed power and dimensions, it is therefore necessary to increase the thrust or pull of the propeller.
It is a particular object of the present invention to provide a blade for shrouded propeller, of which the geometry of the aerodynamically active part is optimalized so that the propeller delivers a thrust or a pull which is as great as possible, whilst consuming a power which is a low as possible for drive thereof.